1. Field of the Invention
The present invention relates to a nickel powder which is, in particular, suitable for forming electrodes in multilayer ceramic electronic components, such as multilayer capacitors, multilayer inductors, and multilayer actuators, to a conductive paste comprising the nickel powder, and a multilayer ceramic electronic component using the same.
2. Description of the Related Art
A multilayer ceramic electronic component (also referred to hereinbelow as “multilayer electronic component”) is usually manufactured in the manner as follows. A ceramic starting material powder such as a dielectric, magnetic, or piezoelectric powder is dispersed in a resin binder and molded into sheets to obtain ceramic green sheets (referred to hereinbelow as “ceramic sheets”). A conductive paste for an internal electrode that is prepared by dispersing an inorganic powder comprising an electrically conductive powder as the main component and optionally a ceramic powder or the like in a vehicle comprising a resin binder and a solvent is printed according to a predetermined pattern on the ceramic sheet and dried to remove the solvent and form a dry film of the inner electrode. A plurality of ceramic sheets each having the dry film for the inner electrode that were thus obtained are laminated and pressurized to obtain a non-fired laminate in which the ceramic sheets and paste layers of inner electrodes are alternately laminated. The laminate is cut to a predetermined shape, then subjected to a binder removal process in which the binder is burned and dissipated, and fired at a high temperature whereby sintering of the ceramic layers and formation of the inner electrode layers are conducted simultaneously and a ceramic body is obtained. Terminal electrodes are then fused to both end surfaces of the body and a multilayer electronic component is obtained. The terminal electrodes and the unfired multilayer body are sometimes co-fired.
In recent years, powders of base metals such as nickel and copper are mainly used instead of powders of noble metals such as palladium and silver as electrically conductive powders of conductive pastes for inner electrodes. Accordingly, firing of the laminate is usually carried out in a nonoxidizing atmosphere with an extremely low partial pressure of oxygen in order to prevent the oxidation of the base metal during firing.
As a demand for miniaturized, highly multilayered electronic components has been growing in recent years, a rapid transition has been made to reducing the thickness of layers in both the ceramic layers and the inner electrode layers, in particular, in multilayer ceramic capacitors using nickel as an electrically conductive powder. As a result, ceramic sheets of smaller thicknesses came into use and also extremely fine nickel powder with a particle size of 1 μm or less, and even 0.5 μm or less, came into use for conductive pastes for inner electrodes.
However, such extremely fine nickel powders are easily sintered, the nickel particles are over-sintered during firing of the capacitor, voids appears in the inner electrodes due to grain growth, and a discontinuous film is obtained, causing increases in electrical resistance and disconnection. In addition, the electrode thickness increases, placing a limitation on the possible film thickness reduction. Furthermore, because the starting temperature of sintering is extremely low and sintering is started at an early stage during firing and also because volume expansion and shrinkage are induced by a redox reaction, the sintering shrinkage behavior does not match that of the ceramic layer, thereby causing structural defects such as delamination or cracking, which results in a lowering of yield and reliability.
In order to resolve such a problem, for example, Patent Document 1 discloses a nickel powder having a mean particle size of 0.1 to 0.8 μm and an oxygen content of 0.5 to 5.0 wt. % and is subjected to surface oxidation. Further, Patent Document 2 discloses a nickel powder having an oxide surface thereon with a certain thickness and this document states that using such a surface-oxidized nickel powder raises the starting temperature of sintering shrinkage, and prevents delamination and cracking as well as the increase in resistance caused by over-sintering. However, the study conducted by the inventors demonstrated that although an oxidized layer formed on the nickel powder surface, such as described in Patent Documents 1 and 2, is effective in preventing structural defects and an increase in the resistance value, the effect thereof is sometimes insufficient. In particular, problems associated with the decrease in capacitor characteristics, occurrence of structural defects, and decrease in reliability are sometimes encountered apparently due to incomplete decomposition of vehicle components in the binder removal process. Thus, in a nonoxidizing atmosphere such as a nitrogen atmosphere used in the binder removal process during firing, a nickel powder, which inherently has a high catalytic activity, acts as a catalyst for decomposition of the resin binder and tends to accelerate the decomposition process. However, if the mean particle size of the nickel powder is on the order of a submicron level, in particular, becomes 0.5 μm or less, the activity of the nickel powder itself further increases and part of the resin sometimes explosively decomposes at a temperature lower than the usual resin decomposition temperature, even when using the nickel powder subjected to surface oxidation by the method described in Patent Documents 1 and 2.
When a resin starts decomposition at a comparatively lower temperature, as described hereinabove, in a nonoxidizing atmosphere, then the resin is not completely decomposed and a carbonaceous residue that was left unburned is intertwined, for example, forming a graphite-like three-dimensional structure, and can hardly be dissipated. As a result, carbon remains in the inner electrode layer after the binder removal process, and when this residual carbon is oxidized in the subsequent process of ceramic sintering at a high temperature, gasified, and dissipated, it pulls oxygen out of the ceramic layer, thereby decreasing the strength of the ceramic body and also degrading electrical characteristics such as electrostatic capacity and insulation resistance. Furthermore, the residual carbon brings the melting of the nickel powder to a lower temperature side, thereby causing over-sintering and degrading the continuity of electrodes. In addition due to explosive decomposition of the resin, structural defects such as cracks sometimes occur in the body, and the properties and reliability of the electronic component are decreased. Therefore, although the catalytic activity of nickel powder is somewhat decreased when an oxidized layer is present on the nickel powder surface, as described in Patent Documents 1 and 2, the increase in the amount of residual carbon and the occurrence of structural defects caused by the above-described decomposition of the resin at a low temperature cannot be completely inhibited.
Patent Document 1: Japanese Patent Publication No. 10-106351 A.
Patent Document 2: Japanese Patent Publication No. 2000-45001 A.